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fp-ts-errors

Handle errors as values using fp-ts Either and TaskEither for cleaner, more predictable TypeScript code. Use when implementing error handling patterns with fp-ts.

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@sickn33

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fp-ts-errors - Practical Functional Error Handling with TypeScript

Skills Overview


Use fp-ts’s Either and TaskEither to treat errors as values. This replaces try/catch with type-safe functional patterns, making TypeScript code more concise, clearer, and more predictable.

Applicable Scenarios

  • API and Service Development

    • When your API or service needs clearly defined error types, Either makes errors visible in the type system. There’s no more guessing about what exceptions a function might throw—TypeScript will force you to handle all possible error cases.

  • Form Validation

    • When you need to collect multiple validation errors rather than returning only the first one, fp-ts’s Validation mode can return all problematic fields at once, improving the user experience.

  • Asynchronous Workflow Orchestration

    • When multiple asynchronous operations must be chained and each step can fail, TaskEither’s fluent chaining makes the control flow easy to follow. It stops automatically at the first error, without needing nested try/catch blocks.

    Core Features

  • Type-Safe Error Handling with Either

    • Either contains either an error (Left) or a successful value (Right). TypeScript’s type system forces callers to handle both cases, eliminating forgotten try/catch. It supports rich combinators such as map, chain, and filterOrElse.

  • Asynchronous Error Handling with TaskEither

    • TaskEither is the asynchronous version of Either, perfectly replacing the Promise + try/catch approach. It supports common async patterns like parallel execution, retry mechanisms, fallback strategies, and more—while keeping everything type-safe.

  • Accumulating Error Validation

    • Using getApplicativeValidation lets you accumulate all validation errors instead of returning after the first failure. It’s especially suitable for form validation scenarios, so users can see all issues that need to be fixed at once.

    Common Questions

    What’s the difference between fp-ts Either and TaskEither?

    Either is for synchronous operations and directly contains either an error or a success value. TaskEither is for asynchronous operations; fundamentally, it’s a function that returns a Promise. TaskEither has all the same operators as Either (map, chain, fold, etc.), but when executed it returns a Promise. In short: use Either for sync and TaskEither for async.

    How do I use fp-ts to collect all form validation errors?

    Use E.getApplicativeValidation together with sequenceS. An ordinary Either chain stops at the first error, but the Validation mode continues running all validations and collects all errors. Pairing with NonEmptyArray ensures there’s at least one error, making the types safer.

    Why use Either instead of try/catch?

    Traditional exceptions are not visible in a function’s type signature. A function that claims to return a User might actually throw three different exceptions. Either makes errors part of the type, so TypeScript forces you to handle all cases. Additionally, Either supports functional composition: you can use pipe to elegantly chain multiple potentially failing operations, resulting in clearer code and easier testing.